2.1. Introduction
The use of CO2 as a chemical feedstock or an oxidant is an appealing strategy for reducing greenhouse gas emissions especially if technologies currently being developed to remove CO2 from fossil fuel fired power plant exhaust gases lead to abundant, high purity, carbon dioxide feedstocks. If the CO2 gas streams can be used as reactants in processes which yield more energetic products, such as fuels or value-added intermediates, then the original carbon in the fossil fuel would be recovered for utilization in another application. Exemplary pathways exist for converting carbon-dioxide to products which can be used in the energy industry for fuel or by the chemical industry for chemical feedstock. These include char gasification to make carbon monoxide from carbon dioxide and carbon, carbon dioxide methanation to make methane from carbon dioxide and hydrogen, and carbon dioxide reforming to make carbon monoxide and hydrogen from carbon dioxide and methane. See, for example Kolb and Kolb, 1983, J Chem Ed 60(1) 57-59 “Organic Chemicals from Carbon Dioxide.” Others have reported on studies of CO2 as a reagent for organic synthesis. See Colmenares, 2010, Current Organic Synthesis 7(6) 533-542 “Novel Trends in the Utilization of CO2 as a Reagent and Mild Oxidant in the C—C Coupling Reactions.” The potential for the upgrading of carbon dioxide through industrial processes has been investigated for over the past one hundred years.
Specifically, U.S. Pat. No. 4,185,083, Walker, discloses a process using the Boudouard Reaction to produce finely divided carbon. U.S. Pat. No. 4,496,370, Billings, and U.S. Pat. No. 4,382,915, Sadhukhan and Billings, disclose a zinc oxide-char gasification process. U.S. Pat. No. 7,259,286, Jothimurugesan et al. disclose iron oxide catalysts for carbon monoxide hydrogenation reactions such as Fischer-Tropsch reaction. The contents of the above are hereby incorporated in its entirety.
Towards these uses certain iron-based materials have been reported due to the high reactivity of reduced iron for oxidation. For example, Tada et al. disclose Fe-valve metal-Pt group elements (including Ru) alloys activated by hydrofluoric acid (HF) for the conversion of CO2 and H2 to methane (methanation of CO2). Tada, et al., AMORPHOUS FE-VALVE METAL-PT GROUP METAL ALLOY CATALYSTS FOR METHANATION OF CO2. Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 1994, 182, 1133-1136.
Recently, Coker et al. reported iron oxide supported on zirconia or yttria-stabilized zirconia (YSZ) for the solar thermal production of hydrogen from water or CO from CO2. Coker et al. J. Mat. Chem 2012 22 6726-6732.